1. Field of the Invention
The invention relates to improved melt extrusion processes and the films produced thereby. The invention relates especially but not exclusively to melt extrusion processes designed to provide improved operating conditions and/or improved film properties.
2. Background Information
Melt processing conditions are conventionally thought to be constrained by phenomena such as draw resonance which leads to a variation in the draw ratio; sharkskin or surface melt fracture; or breaking of the molten portion of the film connecting the solidified film portion and the die gap. These phenomena have generally led to a limitation of the melt processing speed, of the thickness and of the degree of orientation which could be achieved in the resulting films.
The industry has tried to alleviate these problems in a variety of ways including the inclusion of additives to improve surface melt fracture, the blending with a branched polymer such as LDPE and other polymeric or oligomeric components, as well as by modification of the extrusion apparatus using additional draw off rollers.
One route to improve draw resonance behavior is to provide the polymer itself with more elastic behavior by branching or modification of the polymer as described for example in Dow U.S. Pat. No. 5,272,236, U.S. Pat. No. 5,278,272, and EP-608369-A1. Low draw ratios are however used.
Ways of avoiding the various adverse processing phenomena are described in our co-pending patent application WO93/31348.The disclosure addresses ways of avoiding unstable draw resonance zones by appropriate control of the Draw or stretch ratio, Dr, and of the Aspect ratio, A. WO93/31348 did not disclose how additional control can be exercised to place basically non- or low elasticity molten polymer fluids into regions where they exhibit more elastic behavior.
All the art disclosed in the above prior published applications and patents appears to be of lesser relevance, and is incorporated herein by cross reference and will not be discussed in detail herein.
The invention provides means of reaching and operating in an process envelope where the polymer melt acts elastically, even when the polymer is initially of a non- or low-elasticity type. The novel operating envelope which can be reached enables polymer films to be made which have surprising physical properties and can be made at high line speeds.